Add advanced document highlight to the ruby-lsp

[vinistock]

Basic document highlight #87 followed what currently exists for Ruby, which is just highlighting simple occurrences of the same text without doing scope resolution.

Advanced document highlight means that only occurrences of the same entity should be highlighted. This means that if some name is shadowed, we still only highlight the correct instances of the entity.

Example

def foo
  a = 1 # <<- clicking on `a` here should highlight only the last `a` on this method, but not the block argument, which shadows the variable name

  [1, 2, 3].each do |a| # <<- not highlighted
    puts a # <<- not highlighted
  end

  a + 2 # <<- highlighted
end